Brian, this isn't really LBE physics. This is more broilerless gas oven physics. LBEs involve carefully channeled, fast moving, hot gas flowing up and around and over the top of the pizza. In an LBE scenario, convection is critical. In this scenario, it's just rising hot gas that's collecting in the head space of the lid. From that perspective, it doesn't matter all that much if this rising heat is collecting in a tall dome or a low one.
With a lower ceiling, IR is the bigger player here, by far. Thermal mass isn't all that critical for radiative heat. Some mass helps, but as long as the material can stay hot it will give off plenty of radiative heat.
A lower ceiling will provide both a convective and a radiative payload, but, between the two, the IR will be the far major player.
Now, one thing I did notice was your recommendation of 'steel sheet.' Because I think a little mass helps, I'd go with 1/8" steel plate. It's very possible, though, that mass isn't even necessary. In theory, 1/2" steel at 600 should radiate just as much heat as 1/32".